1. Field of the Invention
The present invention relates to a developing device for an image forming apparatus and more particularly to a bearing seal structure for stopping a developer or toner in a bearing portion included in a developing device.
2. Description of the Background Art
Today, the grain size of a developer or that of toner for use in the developing device of an image forming apparatus is decreasing for enhancing image quality. To cope with such a small grain size, a structure for sealing a bearing where toner, for example, is apt to leak to the outside has been proposed in various forms in the past. In one type of seal structure, a so-called V-ring, including an elastic seal lip, is simply fitted on a shaft that extends through a bearing case. More specifically, a V-ring, which is a specific form of a seal ring, is formed of rubber and provided with a generally V-shaped section including a body to be fitted on a shaft and an elastic seal lip positioned at one side of the body in the axial direction of the shaft.
In a seal structure of the type described above, grease is sometimes coated on the surface of a retainer, which the V-ring slidingly contacts, in a thin layer in order to prevent toner from leaking and to obviate noise ascribable to friction between the V-ring and retainer. Although the grease is coated in a thin layer so as not to be introduced in a developer, the amount of the grease is too small to preserve the effect of the grease over a long period of time. Further, it is likely that a developer contacts the grease and is mixed therewith because it is coated on the retainer. Moreover, the V-ring cannot sufficiently exhibit the expected sealing ability when it comes to toner having a small grain size, causing the toner to enter the sealing structure via the V-ring.
In light of the above, a G-seal may be used in combination with a V-ring. A G-seal is another conventional seal ring formed of rubber and having a generally G-shaped section that includes a body and an elastic seal lip formed integrally with the inner periphery of the body. The G-seal seals the outer periphery of a shaft by pressing it with the seal lip in the radial direction. The problem with this configuration is that toner passed through the V-ring adheres to a seal portion due to frictional heat generated between the G-seal and the shaft. Such toner grows in the form of masses and brings about defective images, locking and other problems when introduced into a developer via the seal portion.
The problems mentioned above arise little in a low-speed and a medium-speed image forming apparatus whose drive shafts rotate at speeds of, e.g., 315 rpm (revolutions per minute) and 411 rpm, respectively. However, when such a seal structure is applied to a high-speed image forming apparatus whose drive shaft. rotates at a speed as high as about 468 rpm, the above problems are apt to arise because the V-ring or the G-seal and the shaft of the retainer, frictionally contacting each other, generate a large amount of heat. For example, when a developing device included in a high-speed apparatus is continuously driven, the developing device is heated to about 50° C. with the result that the seal portion is apt to locally exceed 70° C., which is the softening point of toner, when heated.
To solve the problems stated above, Japanese Patent Laid-Open Publication No. 12-250309 proposes a bearing seal structure in which grease is sealed between a V-ring and a G-seal. This bearing seal structure, however, has a problem to be described later left unsolved.
On the other hand, Japanese Patent Laid-Open Publication No. 2001-125374 discloses a bearing seal structure including a seal portion in which a first and a second seal member, each having a respective elastic seal lip, contact the outer periphery of a shaft. Grease is sealed between the two seal members. The bearing seal structure, according to the above document, stably reduces slide loads and exhibits a desirable sealing effect and durability. Although this kind of structure has some advantages to be described later specifically, it is desirable to stably maintain the advantages over a long period of time.